This invention relates to an electronic musical instrument, and more particularly an improvement of an electronic musical instrument of the type wherein a frequency signal representing frequency corresponding to a note name of a depressed key is formed and a musical tone corresponding to the depressed key is produced by utilizing the frequency signal.
As is well known in the art, in many types of electronic musical instruments, a plurality of frequency signals of predetermined frequencies are formed simultaneously. In a first example, a plurality of tone production channels are provided and the tone production of a musical tone relating to depressed keys is assigned to available one or more of the tone production channels. In each tone production channel a plurality of frequency signals are formed corresponding to the tone pitches of the depressed key or keys assigned to the channel. According to a second method, a plurality of frequency signals corresponding to respective note names C through B are provided in advance, and a musical tone is produced by utilizing one or ones relating to the depressed key or keys among these frequency signals.
To this end a system has been proposed wherein a plurality of frequency division channels (a series of frequency dividing circuits) are provided corresponding to respective tone production channels or 12 note names C through B, and the frequency of a clock pulse produced by a main oscillator is divided at a predetermined frequency division ratio in each frequency division channel to produce frequency signals. For example, the first method is disclosed in Japanese Preliminary Patent Publication No. 3257/1978 dated Feb. 4, 1978, and the second method is disclosed in U.S. Pat. No. 3,818,354 dated June 18, 1974.
Since in these prior art systems, each frequency division channel is constituted by a first counter which divides the frequency of the clock pulse at a frequency division ratio determined by a combination of frequency division ratios N and N+1 (N is a positive integer) and a second counter for counting the number of the frequency division operation by the first counter and for transfering or switching the frequency division ratio of the first counter between the ratios N and N+1 according to the number of the frequency division operations, it was necessary to provide two counters for each frequency division channels thereby complicating the construction and enlarging the electronic musical instrument.
In the first method, a frequency number (numerical data) corresponding to the tone pitch of the depressed key is sequentially accumulated and the accumulated value (waveform generating data) is applied to a waveform memory device as an address signal to cause the memory device to produce a musical tone signal corresponding to the tone pitch of the depressed key. With this method, however, the reading out of the musical tone signal waveform from the waveform memory device is initiated just after the completion of the tone production assignment relating to the depressed key. Therefore, where notes relating to keys of the same note of different octaves are assigned to two tone production channels, due to the difference in the depression times of these two keys the musical tone signal waveforms of the two channels would have opposite phases with the result that the tones produced by the two channels will be canceled each other, meaning that a phase difference between tones having the same note name of different octaves distorts the musical tone. The same problem also occurs between keys of the same note name of the upper and lower keyboards.
In the former method, a waveform memory device is used for respective tone production channels, on a time division basis. In this case, since the operating frequency, that is frequency which the specified one of the time division time slots occurs, is set independently of the frequencies of the musical tones to be produced by respective tone production channels, the musical tone signal waveform formed on the time division basis would contain a clock component and a unwanted reflected noise component which not only distorts the waveform but also makes the produced sound unclear.
According to an electronic musical instrument disclosed in U.S. Pat. No. 4,228,403, number of the frequency signals corresponding to note names C through B are counted to form a serial pulse train in which weights of 2.sup.0 to 2.sup.n (n is a positive integer) are assigned to respective note names C through B, the pulse train being produced for discrete note names as frequency divided data (waveform generating data) obtained by sequentially dividing the frequency of the frequency signal by 1/2. On the utilization side (tone production channels) frequency divided data corresponding to the note name of the depressed key are selected out of a group of frequency divided data for discrete note names, and a one bit signal having a weight corresponding to the octave tone range of the depressed key is selected from the selected frequency divided data so as to utilize the selected one bit signal as a musical tone signal regarding the depressed key.
In the electronic musical instrument of this type no phase difference occurs between notes of the same note name of different octaves or notes of the same note of different keyboards as above described. However, as it is necessary to form the pulse train for each of the note names, it is necessary to provide a number of counters having many stages for forming the pulse trains. Furthermore, in each tone production channel, as it is necessary to receive all frequency divided data groups for discrete note names as inputs and to select one frequency divided data regarding a note name of a musical tone to be produced out of the inputs, it is necessary to use a selector having a large number of input bits thus increasing the scale of the circuit.